The present invention relates to charge transfer devices, and in particular to a charge transfer device of the type in which the charge transfer channel is buried in a semiconductor body of a conductivity type opposite to the conductivity type of the transfer channel.
Many efforts have recently been directed to development of charge transfer devices of the buried channel type in which potential "wells" are produced in response to application of different potentials to electrodes mounted over the channel. Such devices are typically intended for use as delay elements for video information transfer or as a solid state image sensor instead of surface charge transfer type devices. In particular, the use of the channel type devices as a buried image sensor is advantageous because it allows a self-scanning circuitry to be formed on a common semiconductor substrate with charge tranfer channels. However, when applying the charge transfer device as a solid image sensor, the number of potential wells per unit area determines the image quality. An effort for providing as many potential wells as possible in a given space of a semiconductor body has encountered limitations on the maximum quantity of electrons available for transfer to adjacent potential wells because of the undesirable lateral field effect of the potential which occurs in the neighborhood of a boundary with the semiconductor body. The charge transfer channel could have an increased impurity concentration to provide a deeper potential well to accommodate a greater number of electrons therein. However, a higher driving voltage is required to transfer all the electrons from one potential well to the next. Thus, the prior art charge transfer device of the buried channel type is poor in signal-to-noise performance and thus unsatisfactory for solid-stage image sensors.